1. Field of the Invention
The invention relates to semiconductor devices and semiconductor device fabrication, and in particular a method of forming a quantum well semiconductor device demonstrating transistor action based upon the quantum properties of electrons in a narrow quantum well, and devices made thereby.
2. Description of the Prior Art
There are a variety of different types of semiconductor devices known in the prior art. One of the more general methods of classifying these devices is in respect to the physical basis for device operation, in which there are three categories:
1. charge variation in the gate capacitor (field-effect transistors or FETs); PA0 2. variation of a potential barrier for charge injection (e.g., bipolar, permeable base, hot electron transistors); and PA0 3. variation of electron temperature (CHINT), as described in A. Kastalsky, J. M. Abeles, R. Bhat, W. K. Chan and M. A. Koza, Appl. Phys. Lett. 48, 71, (1986)
In gallium arsenide technology, a number of these different types of semiconductor devices, and semiconductor device structures have been investigated and are known in the art. GaAs MESFET devices, for example, have been quite successful in applications such as microwave and high speed digital circuits. This success of GaAs MESFET's has been due mainly to high electron velocity of GaAs and the commercial availability of semi-insulating GaAs substrates for device fabrication. Some of the drawbacks of GaAs MESFET technology are difficult circuit modeling and design, poor threshold voltage control, and the sensitivity of MESFET circuit operation to load conditions.
Bipolar technology is well known for its advantages in terms of uniform threshold, noise immunity, high speed and high current drive. A conventional bipolar transistor has an npn or pnp structure wherein emitter, base, and collector layers are made of a common semiconductor material. In this case, emitter and collector junctions are homojunctions.
Bipolar transistors using a heterojunction as the emitter-base junction are receiving a great deal of attention and are being extensively studied these days. The heterojunction bipolar transistor with the emitter energy gap wider than that of the base has an advantage in that, when the emitter junction is forward biased, carriers can be easily injected from the emitter to the base while carrier injection from the base to the emitter is limited due to an energy gap difference between emitter and base layers. Therefore, a current gain of the heterostructure bipolar transistor becomes higher than that of the conventional homostructure type.
In all of the semiconductor device structures of the prior art, none is based on quantum properties of the materials or structures. In other words, the basic transistor action in prior art devices is based upon and can be satisfactorily described without reference to any quantum effect. Although quantum effects may influence device performance in the prior art (for example, mobility enhancement in the 2-dimensional channel of the modulation doped FET or MODFET) or bring about useful device modification such as a tunnel emitter in the hot electron transistor, (M. Heiblum, D. C. Thomas, C. M. Knoedler and M. I. Nathan, Appl. Phys. Lett. 47, 1105, 1985) or in the quantum well base in the induced-based transistor (S. Luryi, IEEE Electr. Dev. Lett. EDL-6, 178, 1985), prior to the present invention a device has not been constructed in which the transistor action itself is based on the quantum properties of the device structure.